"Reinhard Zwirner" wrote in message
...

IIRC, a diac is like a triac with no gate - when it fires the dropped
voltage is a couple of volts only. This is unlike a zener.

You're right: but would there be a difference in function between a
triac triggered (at 30 V) by a diac and a triac triggered (at 30 V) by
two back-to-back zeners? Wouldn't both of them conduct current until its
next zero crossing?

The point of the diac is to dump enough current into the gate to turn on the
triac. A zener doesn't do that - it just passes the current once the voltage
is exceeded.